This invention relates to optical transducers, and more particularly to optical transducers for determining liquid level within tanks and the like.
Transducers for measuring liquid level are often used in vehicles, industrial equipment and other stationary and mobile systems. The electrical output of such transducers change in response to a change in the liquid being measured, and is typically in the form of a change in resistance, capacitance, current flow, magnetic field, frequency, and so on. These types of transducers may include variable capacitors or resistors, optical components, Hall Effect sensors, strain gauges, ultrasonic devices, and so on.
By way of example, prior art liquid level sensors, such as fuel level sensors for motor vehicles, usually include a float that rides on an upper surface of the fuel in a fuel tank. The float is typically connected to one end of a pivot arm while the other end of the pivot arm includes a wiper mechanism that brushes against a resistor strip when the arm is rotated due to a change in fuel level in the tank. Such sensors are prone to wear, mechanical and/or electrical breakdown, as well as inaccurate liquid level detection.
In an effort to overcome these drawbacks, liquid level transducers with no moving parts have been developed. Such transducers include heated wires or thick film devices wherein the liquid being measured functions as a heat sink, variable capacitance devices wherein the measured liquid functions as the dielectric, ultrasonic devices, optical devices, and Hall-Effect devices. Many of these liquid level transducers suffer from one or more of the following disadvantages: the detection of liquid level may be inaccurate; fine wires can be affected by vibration and movement, such as with heated wire probes; fuel additives and contaminants can cause corrosion of various transducer components; the electrical output of the transducer may vary due to water or contaminants in the fuel, such as with capacitance-type probes; and such transducers may be cost-prohibitive in many applications.
With respect to optical-type liquid level transducers, it has previously been difficult to economically construct an optical liquid level sensor for fuels or other liquids, due to the relatively high cost of materials that are resistant to the liquids being measured as well the refractive index of some optical materials, as they tend to be higher than the refractive index of the liquids. By way of example, a low-cost material such as acrylic, although resistant to many liquids, has a refractive index of approximately 1.49, while water has a refractive index of approximately 1.32 and unleaded gasoline has a refractive index of approximately 1.38. When a cylindrical probe constructed of acrylic is positioned in the liquid, any light entering through one end the probe will typically be totally internally reflected along the probe length. Accordingly, the light output from the probe will generally be constant and thus fail to generate a varying signal to be used for liquid level measurement. Materials with a refractive index lower than the liquid to be measured may suffer from other drawbacks, such as high attenuation of light through the material.